Nuclear magnetic resonance (NMR) diagnostic imaging systems are based on nuclear phenomena which occur when atomic nuclei located in a static, uniform magnetic field, are stimulated by a second magnetic field rotating at the Larmor frequency associated with particular nuclei. Upon removal of the rotating stimulation field, the stimulated nuclei relax and emit the absorbed energy in the form of radio frequency signals, termed NMR signals, which are received and processed to provide a visible display of the nuclei.
As essential requirement in NMR imaging is obtaining a uniform magnetic field in a region termed the "imaging sphere". It is within this sphere that a medically significant portion of a patient is positioned for the purpose of obtaining an NMR image of such portion. Typically, a suitable uniform magnetic field is obtained using large electromagnets that require cooling the current carrying conductors with liquid helium to a temperature near absolute zero. The cooling required with conventional electromagnetics contribute to the expense and complexity of an NMR installation.
The use of permanent magnets would greatly simplify installation and operation of an NMR installation; but the problem is how to establish a field of the necessary strength and uniformity within the image sphere. An article by K. Halback entitled "Design of Permanent Magnet Multipole Magnets with Oriented Rare Earth Cobalt Materials" appearing in the Nuclear Instruments and Methods (Vol. 169, p. 1-10, 1980), hereinafter referred to as Reference [1], provides important information in terms of materials and geometry for a first step in substituting a permanent magnet for a conventional electromagnet in NMR imaging device.
In the article, the author describes rare earth cobalt (REC) material whose properties approach those of an ideal magnet: permeability of unity and zero susceptibility. REC material, and to a lesser extent the ferrites, are magnetically hard, have a high coercivity, and thus resist external magnetic influences. The author applies REC material to the design of multipole magnets, and particularly quadrapoles for use in beam optics (i.e., the technology by which particle beams are focused) and discusses the theory by which mutlipole permanent magnets can be designed. Thus, the article derives the relationship between the easy axis of magnetization in an infinitely long cylinder of material having perfect magnetic properties and having an axial aperture, and the field within such aperture.
While the author is interested in quadrapoles, it is clear that a dipole magnet (which produces a uniform transverse field within the aperture) would be one in which the angular variation of the easy axis is 2.theta.. That is to say at 0.degree., the easy axis of magnetization would be 0.degree.; at 45.degree., th easy axis orientation would be 90.degree., etc. The relationship set forth in Reference [1] requires perfect magnetic material, a continuous variation in easy axis orientation as a function of angle, and an infinite axial extent. In such an arrangement, it can be shown that the field B.sub.0 inside an aperture of radius r.sub.1 in an infinitely long cylinder of outer radius r.sub.2 is B.sub.r [1n (r2/r1)] where B.sub.r is termed the remanent magnetization. The field outside the cylinder is zero.
Reference [1] also applies these theoretical principles to real materials wherein the intrinsic coercivity is finite such that the magnetization of the material is affected by external fields, the material is not homogeneous, the easy axis of magnetization is a discrete rather than a continuous function of .theta., and the axial extent is finite. As a consequence of these practical limitations on both material and geometry, the author of Reference [1] finds that distortions in the ideal field are introduced by using a magnet composed of a plurality of segments, each having a fixed easy axis of magnetization throughout. Specifically, the author shows that the introduced distortion is harmonic with spatial frequencies proportional to the number of discrete segments, and shows that properly spacing the segments can remove the fundamental from the distortion.
Reference [1] also discusses the fringe fields that develop at the end of multipole magnets by reason of their finite length. Essentially, the author takes the position that a magnet of finite length can be analyzed as two semifinite lengths of opposite sign.
The problem with directly applying the teachings of Reference [1] to the problem of obtaining a dipole field for an NMR imaging system is field nonuniformity. The field within a sphere at the center of a cylindrical magnet of reasonable length constructed in accordance with Reference [1] is not sufficiently uniform for NMR imaging even when conventional shimming coils are employed. Therefore, an object of the present invention is to improve the field uniformity within a predetermined region in a cylindrical permanent magnet arrangement.